Wireless communication apparatus

ABSTRACT

A wireless communication apparatus includes a plurality of antennas, transmits a plurality of known symbol sequences each including a plurality of known symbols by using the antennas, each of the known symbols having subcarrier arrangement on which plural known information of the each of the known symbols are carried on, and inversion/non-inversion of a phase of known information on one of two adjacent subcarriers of the subcarrier arrangement being controlled in accordance with the number of the known symbols and time position of the each of the known symbols, and transmits data symbols by using the antennas after the known symbol sequences are transmitted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-171669, filed Jun. 9, 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to Multi-Input Multi-Output OrthogonalFrequency Division Multiplexing (MIMO-OFDM) for communication using aplurality of antennas and a plurality of subcarriers, and also relatesto the technique of a high-speed wireless LAN.

2. Description of the Related Art

In the conventional wireless LAN (802.11a), synchronous processing andchannel estimation are performed by transmitting known symbols (a shortpreamble and long preamble) before a data field. By using thesepreambles, the subsequent signal and data can be demodulated. Recently,a high-speed wireless LAN standard called IEEE802.11n is beingdiscussed. To achieve a transmission rate of 100 Mbps in a MAC layer,the IEEE802.11n is based on MIMO (Multi-Input Multi-Output) using aplurality of antennas. When the conventional preamble structures of thiswireless LAN are to be applied to the MIMO system, therefore, thearrangements of a short preamble and long preamble must be changed tothose for MIMO.

In a preamble structure proposed in a reference (Jan Boer and two otherpersons, “Backwards compatibility”, [online], September 2003, IEEE LMSC(publisher), [searched Sep. 15, 2003], Internet <URL:ftp://ieee.wireless@ftp.802wirelessworld.com/11/03/11-03-0714-00-OOOn-backwards-compatibility.ppt>),a short preamble sequence used for time synchronization, frequencysynchronization, and automatic gain control (AGC), a long preamblecontaining a symbol for estimating a channel response, and a signalfield are first transmitted from one transmitting antenna, and then longpreambles for estimating channel responses are transmitted in turn fromother transmitting antennas. After the transmission of the preamble iscompleted, data is simultaneously transmitted from a plurality oftransmitting antennas. That is, long preambles for channel responses aretransmitted from a plurality of transmitting antennas by time-divisionmultiplexing.

In the MIMO system, the receiver must estimate the number oftransmitting antennas in order to demodulate a transmission sequence. Ifthis estimation of the number of transmitting antennas fails, thesubsequent data field cannot be demodulated any longer. Therefore, theestimation requires very high accuracy. As a method by which thereceiver estimates the number of transmitting antennas, it is possibleto transmit a signal notifying the number of transmitting antennas fromthe transmitter. In this method, however, the overhead increases, andthis unavoidably lowers the throughput of data transmission. It is alsopossible to estimate the number of transmitting antennas by using areceived preamble signal. Since the preamble signal of the abovereference is not for estimating the number of transmitting antennas, itis difficult to estimate the number of transmitting antennas by usingthis preamble.

In the MIMO system as described above, if the estimation of the numberof transmitting antennas fails, the subsequent data portion cannot bedemodulated any longer, so the estimation requires very high accuracy.On the other hand, the method by which the transmitter transmits asignal which notifies the number of transmitting antennas has theproblem that the overhead inevitably increases. Also, in the techniquedescribed in the above reference, it is difficult to estimate the numberof transmitting antennas by using the preamble signal.

It is, therefore, an object of the present invention to provide awireless communication apparatus by which the receiving side can easilyestimate the number of transmitting antennas used in transmissionwithout any addition of a signal for notifying the number oftransmitting antennas on the transmitter, and as a consequence a datasymbol can be correctly demodulated.

BRIEF SUMMARY OF THE INVENTION

According to embodiments of the present invention, a wirelesscommunication apparatus comprises a plurality of antennas; transmits aplurality of known symbol sequences each including a plurality of knownsymbols by using the antennas, each of the known symbols havingsubcarrier arrangement on which plural known information of the each ofthe known symbols are carried on, and inversion/non-inversion of a phaseof known information on one of two adjacent subcarriers of thesubcarrier arrangement being controlled in accordance with the number ofthe known symbols and time position of the each of the known symbols;and transmits data symbols by using the antennas after the known symbolsequences are transmitted.

According to embodiments of the present invention, a wirelesscommunication apparatus receives a plurality of known symbol sequencesand subsequent data symbols transmitted by a plurality of antennas, eachof the known symbol sequence including a plurality of known symbols,each of the known symbols having subcarrier arrangement on which pluralknown information of the each of the known symbols are carried on, andinversion/non-inversion of a phase of known information on one of twoadjacent subcarriers of the subcarrier arrangement being controlled inaccordance with the number of the known symbols and time position of theeach of the known symbols; calculates each channel estimation valuecorresponding to each of the subcarriers, from each known symbolreceived; calculates a correlation value between two channel estimationvalues corresponding to the two adjacent subcarriers, to obtain eachcorrelation value corresponding to the each known symbol received;estimates the number of antennas based on the each correlation value andthe number of known symbols received; and; processes the data symbolsreceived by using the each channel estimation value and the number ofantennas estimated.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing the arrangement of a transmitteraccording to the first to fourth embodiments of the present invention;

FIGS. 2 to 4 are views for explaining a known symbol transmission methodaccording to the first embodiment;

FIG. 5 is a block diagram showing the arrangement of a receiveraccording to the first to third embodiments;

FIG. 6 is a flowchart for explaining a process for estimating the numberof transmitting antennas in the receiver shown in FIG. 5;

FIGS. 7 to 9 are views for explaining a known symbol transmission methodaccording to the second embodiment;

FIGS. 10 to 12 are views for explaining a known symbol transmissionmethod according to the third embodiment;

FIGS. 13 to 15 are views for explaining a known symbol transmissionmethod according to the fourth embodiment;

FIG. 16 is a block diagram showing the arrangement of a receiveraccording to the fourth embodiment;

FIG. 17 is a view showing an example of a table showing the patterns(known symbol patterns) of known symbols transmitted from individualtransmitting antennas; and

FIG. 18 is a flowchart for explaining a process for estimating thenumber of transmitting antennas in a receiver according to the fifthembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawing. A wireless communication systemaccording to each embodiment is applicable to, e.g., a wireless LAN ormobile communication system (cellular system) which includes at leastone base station apparatus and at least one terminal apparatus. Atransmitter and receiver included in a wireless communication apparatussuch as the base station apparatus or terminal apparatus will beexplained below.

First Embodiment

First, a transmitter according to the first embodiment will be describedbelow with reference to FIG. 1. FIG. 1 shows physical layers of thistransmitter. Data (a bit string) 10 to be transmitted is input incertain transmission units (e.g., frames or packets) from an upperlayer. An encoder 11 performs, e.g., error correction coding on theinput data 10, and generates a coded bit sequence. A serial-to-parallel(S/P) converter 12 divides the coded bit sequence into a plurality ofstreams by serial-to-parallel conversion. Modulators 13-1 to 13-M mapthese streams on a complex plane to generate modulated data symbols.

Serial-to-parallel (S/P) converters 14-1 to 14-M performserial-to-parallel conversion on the modulated data symbols so that theyare transmitted on subcarriers of orthogonal frequency-divisionmultiplexing (OFDM). In addition, inverse fast Fourier transform (IFFT)units 18-1 to 18-M transform these signals on the frequency domain intowaveforms in the time domain. The waveform in the time domain outputfrom the IFFT units 18-1 to 18-M are input to a transmitting unit 19.

In the transmitting unit 19, a guard interval (GI) is added to thesignals output from the IFFT units 18-1 to 18-M, and the obtainedsignals are converted into analog signals by a D/A converter. Thesignals output from the D/A converter are converted (up-converted) intoan RF (Radio-Frequency) band by a frequency converter, and supplied totransmitting antennas 20-1 to 20-M via a power amplifier. The OFDMsignals are transmitted from the transmitting antennas 20-1 to 20-M to awireless communication apparatus of a communication partner.

Preambles are transmitted before the data symbols are thus transmittedas the OFDM signals. A transmission system of known symbols, which areused for estimating channel will be explained below.

A known symbol pattern generator 15 is, e.g., a ROM, and stores aplurality of known symbol patterns. Each known symbol is transmitted bycarrying its information on some of a plurality of OFDM subcarriers. Aknown symbol pattern indicates a subcarrier on which information of aknown symbol is to be carried. In the example shown in FIG. 1, the ROMstores known symbol patterns on the frequency domain.

In the case that a known symbol is to be transmitted, a plurality ofknown symbol patterns stored in the ROM of the known symbol patterngenerator 15 are sequentially read out at the transmission timing of theknown symbol in accordance with a signal from a counter 16. The counter16 counts the time, and outputs the count value which momentarilychanges.

When known symbol patterns on the frequency domain are stored in the ROMof the known symbol pattern generator 15 as in this example, the readoutknown symbol patterns are input to the IFFT units 18-1 to 18-M via aselector 17, converted into waveforms in the time domain, and suppliedto the transmitting unit 19.

If waveforms in the time domain of known patterns are stored in the ROM,readout known symbols are supplied to the transmitting unit 19 bybypassing the IFFT units 18-1 to 18-M.

A plurality of known symbols are continuously transmitted from eachantenna. The selector 17 distributes known symbol patterns read out fromthe ROM of the known symbol pattern generator 15, in accordance with thetransmission timings of the known symbols which are continuouslytransmitted, such that the readout known symbol patterns are transmittedfrom appropriate transmitting antennas. That is, the selector 17distributes the known symbol patterns to the transmitting antennas 20-1to 20-M in accordance with the count value indicating time informationfrom the counter 16. Note that if a plurality of types of known symbolssuch as a short preamble and long preamble included in preambles of awireless LAN are present, the counter 16 and selector 17 selectivelyread out these different types of known symbol patterns from the ROM.

The selector 17 prestores a table as shown in FIG. 17 which showspatterns (known symbol patterns) of known symbols transmitted from theindividual transmitting antennas. On the basis of this table, theselector 17 distributes the known symbol patterns read out from theknown symbol pattern generator 15, such that these patterns aretransmitted from appropriate transmitting antennas. Note that for thesake of simplicity, the transmitting antennas 20-1 to 20-M shown in FIG.1 are represented by antennas 1 to M in FIG. 17.

Referring to FIG. 17, for symbol 1 as one of known symbols transmittedfrom antenna 1, antenna 2, . . . , antenna M-1, and antenna M, pattern1, pattern 2, . . . , pattern M-1, and pattern M are transmitted fromthese antennas. For symbol 2, pattern 2, pattern 3, . . . , pattern M,and pattern 1, which are shifted by one pattern from the known symbolpatterns transmitted from symbol 1, are transmitted as known symbolpatterns from the antennas. Likewise, for symbol M-1, pattern M-1,pattern M, . . . , pattern M-2 are transmitted as known symbol patternsfrom the antennas. For symbol M transmitted last, pattern M′, pattern1′, . . . , pattern M-1′ are transmitted from the antennas. A pattern k′(k=1, . . . , M) carries its information on the same subcarrier ascarrying information of a known symbol for a pattern k, and uses a valuedifferent from the pattern k as the information.

On the other hand, when receiving the M known symbols simultaneouslytransmitted from the individual transmitting antennas, a receiver (to bedescribed later) can obtain channel estimation values for allsubcarriers and can also estimate the number of transmitting antennas.

An example of a method of transmitting known symbols for channelestimation will be explained in detail below with reference to FIGS. 2to 4. FIGS. 2 to 4 illustrate the structures of radio frames containingpreambles when there are, respectively, one, two, and three transmittingantennas which simultaneously transmit known symbols. The firstembodiment assumes a system such as a wireless LAN which transmits ashort preamble SP for synchronization and a long preamble LP for channelestimation before a data field (DATA). The arrangement of the shortpreamble SP is not particularly limited. For example, a short preamblesimilar to the IEEE 802.11a can be transmitted from a plurality oftransmitting antennas.

A known symbol is used to estimate a channel response in MIMOcommunication. In a wireless LAN, a known symbol corresponds to the longpreamble LP shown in FIGS. 2 to 4. Referring to FIGS. 2 to 4,frequency-division multiplexing is performed on the long preamble LPtransmitted from each transmitting antenna. Letting M be the number oftransmitting antennas and N be the number of OFDM subcarriers, assumethat N can be divided by 2M without a remainder. In this case,information of a known symbol exists in subcarriers represented byexpressions (1) and (2) below (the numbers of the N subcarriers aredefined as 0th to (N-1)th), and does not exit in any other subcarrier.2(Mk+m+i−2) mod N  (1){2(Mk+m+i−2)+1} mod N  (2)where m=1, 2, . . . , M are the antenna numbers, i=1, 2, 3, . . . , arethe numbers of known symbols in the time direction, and k=0, 1, . . . ,(N/2M-1).

In addition, as shown in FIG. 2, letting L₁(n) be an information valueof a known symbol carried on the nth subcarrier when there is oneantenna, an information value LM(n) of the ith known symbol carried onthe nth subcarrier of the mth antenna when there are M (≧2) antennas isgiven by

if i=M or n is an even numberL _(M)(n)=L ₁(n)  (3)if i # M and n is an odd numberL _(M)(n)=−L ₁(n)  (4)

For example, in the case of FIG. 2 (M=1: one antenna), the combinationsof the numbers of subcarriers, in which information of one known symboltransmitted from antenna 1 exists, and information values carried onthese subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th} subcarriers, information values are{L₁(0), L₁(1), L₁(2), L₁(3), L₁(4), L₁(5), L₁(6), L₁(7), L₁(8), L₁(9),L₁(10), and L₁(11)}

In the case of FIG. 3 (M=2: two antennas), the combinations of thenumbers of subcarriers, in which information of two known symbolstransmitted from antennas 1 and 2 exists, and information values carriedon these subcarriers are as follows on the basis of expressions (1),(2), (3), and (4).

Antenna 1: first known symbol: in {0th, 1st, 4th, 5th, 8th, and 9th}subcarriers, information values are {L₁(0), −L₁(1), L₁(4), −L₁(5),L₁(8), and −L₁(9)}

Antenna 1: second known symbol: in {2nd, 3rd, 6th, 7th, 10th, and 11th}subcarriers, information values are {L₁(2), L₁(3), L₁(6), L₁(7), L₁(10),and L₁(11)}

Antenna 2: first known symbol: in {2nd, 3rd, 6th, 7th, 10th, and 11th}subcarriers, information values are {L₁(2), −L₁(3), L₁(6), −L₁(7),L₁(10), and −L₁(11)}

Antenna 2: second known symbol: in {0th, 1st, 4th, 5th, 8th, and 9th}subcarriers, information values are {L₁(0), L₁(1), L₁(4), L₁(5), L₁(8),and L₁(9)}

In the case of FIG. 4 (M=3:three antennas), the combinations of thenumbers of subcarriers, in which information of three known symbolstransmitted from antennas 1, 2, and 3 exists, and information valuescarried on these subcarriers are as follows on the basis of expressions(1), (2), (3), and (4).

Antenna 1: first known symbol: in {0th, 1st, 6th, and 7th} subcarriers,information values are {L₁(0), −L₁(1), L₁(6), and −L₁(7)}

Antenna 1: second known symbol: in {2nd, 3rd, 8th, and 9th} subcarriers,information values are {L₁(2), −L₁(3), L₁(8), and −L₁(9)}

Antenna 1: third known symbol: in {4th, 5th, 10th, and 11th}subcarriers, information values are {L₁(4), L₁(5), L₁(10), and L₁(11)}

Antenna 2: first known symbol: in {2nd, 3rd, 8th, and 9th)} subcarriers,information values are {L₁(2), −L₁(3), L₁(8), and −L₁(9)}

Antenna 2: second known symbol: in {4th, 5th, 10th, and 11th)}subcarriers, information values are {L₁(4), −L₁(5), L₁(10), and −L₁(11)}

Antenna 2: third known symbol: in {0th, 1st, 6th, and 7th)} subcarriers,information values are {L₁(0), L₁(1), L₁(6), and L₁(7)}

Antenna 3: first known symbol: in {4th, 5th, 10th, and 11th)}subcarriers, information values are {L₁(4), −L₁(5), L₁(10), and −L₁(11)}

Antenna 3: second known symbol: in {0th, 1st, 6th, and 7th)}subcarriers, information values are {L₁(0), −L₁(1), L₁(6), and −L₁(7)}

Antenna 3: third known symbol: in {2nd, 3rd, 8th, and 9th} subcarriers,information values are {L₁ (2), L₁ (3), L₁ (8), and L₁ (9)}

Note that in FIGS. 2 to 4, the preamble structure is represented in timedomain. However, for the sake of convenience, subcarriers in whichinformation of the long preamble LP exists are represented by obliquelines and dots. Also, subcarriers indicated by dots in FIGS. 2 to 4represent subcarriers into which information whose phase is inverted byexpression (4) is inserted. Furthermore, even when there are four ormore antennas, the numbers of subcarriers where information exists inknown symbols transmitted from the individual antennas and theinformation values are obvious from the above analogy.

As shown in FIGS. 2 to 4, the known symbol according to the firstembodiment uses two adjacent subcarriers, and different subcarriers areused for different antennas. Also, in the known symbol transmitted last,the same information as the known symbol when there is one antenna asshown in FIG. 2 is carried on each subcarrier. In a known symboltransmitted before the last known symbol, the same information as theknown symbol when there is one antenna is carried on even-numberedsubcarriers, and phase-inverted information of the known symbol whenthere is one antenna is carried on odd-numbered subcarriers.

The receiver according to the first embodiment of the present inventionwill be described below with reference to FIG. 5. Referring to FIG. 5,the OFDM signals of the RF band transmitted from the transmitter shownin FIG. 1 are received by a plurality of receiving antennas 30-1 to30-M. OFDM received signals from the receiving antennas 30-1 to 30-M areinput to a receiving unit 31.

In the receiving unit 31, the input OFDM signals from the receivingantennas 30-1 to 30-M are amplified by a low-noise amplifier (LNA), andconverted (down-converted) into a base band by a frequency converter. Inaddition, these frequency-converted signals are converted into digitalsignals by an analog-to digital (A/D) converter, and the guard interval(GI) is removed from the digital signals.

The output signals from the receiving unit 31 are input to fast Fouriertransform (FFT) units 32-1 to 32-M where these waveform signals in thetime domain are transformed into waveform signals in the frequencydomain, i.e., into the waveforms of individual subcarriers. Of theoutput signals from the FFT units 32-1 to 32-M, signals of data symbolsections are input to an MIMO signal processing unit 39.

On the other hand, of the output signals from the FFT units 32-1 to32-M, signals of preambles, particularly, known symbol sections areinput to dividing units 33-1 to 33-M. The waveforms of the individualsubcarriers input to these dividing units are divided by a known symbolpattern stored in a ROM 34, and thereby converted into estimation valuesof the channel characteristics. These estimation values are stored inmemories 38-1 to 38-M and input to correlators 35-1 to 35-M. The knowninformation pattern stored in the ROM 34 is the same as in the case ofFIG. 2 (the number M of antennas is one).

The correlators 35-1 to 35-M calculate a correlation value by using thechannel characteristic estimation values, and input the correlationvalue to a determination unit 36. If the determination unit 36determines that the input correlation value is negative, a counter 37 isincremented, and the next known symbol is received. If the determinationunit 36 determines that the input correlation value is positive, thecurrent counter value is output as an estimation value of the number oftransmitting antennas to the MIMO signal processing unit 39. Details ofthe above transmitting antenna count estimating algorithm will beexplained later.

The MIMO signal processing unit 39 performs an MIMO signal receivingprocess, e.g., maximum likelihood estimation, on the data symbol sectionsignals from the FFT units 32-1 to 32-M in accordance with the channelestimation values from the memories 38-1 to 38-M, and the estimationvalue of the number of transmitting antennas from the counter 37.Channel decoding is performed on the signals having undergone this MIMOsignal receiving process, thereby reconstructing transmitted data 40.

Assume that the nth subcarrier signal of the ith known symbol receivedby the jth receiving antenna is Xj(i,n). An estimation value (an outputsignal from a dividing unit) Aj(i,n) of the channel characteristic ofthis subcarrier is given byAj(i,n)=Xj(i,n)/L1(n)  (5)

Letting hj(i,n) be the actual channel characteristic value of thissubcarrier, and Nj(i,n) be the noise signal, Xj(i,n) can be expressed byXj(i,n)=hj(i,n)·L _(M)(n)+Nj(i,n)Therefore, equation (5) can be expressed byAj(i,n)=hj(i,n)·L _(M)(n)/L1(n)+Nj(i,n)/L1(n)  (6)

To simplify the explanation, assume an ideal environment (Nj(i,n)=0) towhich no noise is added. In this case, equation (6) can be simplyexpressed byAj(i,n)=hj(i,n)·L _(M)(n)/L1(n)  (7)

Assume that the effects of channels between adjacent subcarrierstransmitted from the same antenna are substantially the same. That is,assuming that the channel characteristic of adjacent subcarrierstransmitted from the same antenna have a high positive correlationvalue, it is expected that the following channel characteristic Aj(i,n)is obtained from all the receiving antennas.

<One Transmitting Antenna>

Since LM(n)=L₁(n), Aj(i,n)=hj(i,n) holds from equation (7). Also, sinceall subcarriers are transmitted from the same antenna, an estimationvalue Aj(i,n) of the channel characteristic of an even-numberedsubcarrier and an estimation value Aj(i,n+1) of the channelcharacteristic of an adjacent subcarrier having the next number have ahigh positive correlation.

<Two Transmitting Antennas>

For even-numbered subcarriers of the first received known symbol,L_(M)(n)=L₁(n) holds from equation (3), so Aj(i,n)=hj(i,n) holds fromequation (7). On the other hand, for odd-numbered subcarriers,L_(M)(n)=−L₁(n) holds from equation (3), so Aj(i,n)=−hj(i,n) holds fromequation (7), i.e., the phase of this characteristic is inverted fromthat of the actual channel characteristic. Since an even-numberedsubcarrier and an adjacent subcarrier having the next number aretransmitted from the same antenna, an estimation value Aj(i,n) of thechannel characteristic of the even-numbered subcarrier and an estimationvalue Aj(i,n+1) of the channel characteristic of the subcarrier havingthe next number have a high correlation. However, Aj(i,n+1) correspondsto an odd-numbered subcarrier, and its characteristic has a phaseinverted from that of the actual channel. Therefore, the estimationvalues of these subcarriers have a high negative correlation.

In the second received known symbol, L_(M)(n)=L₁(n) holds from equation(3), so Aj(i,n)=hj(i,n) holds from equation (7). Since an even-numberedsubcarrier and an adjacent subcarrier having the next number aretransmitted from the same antenna, an estimation value Aj(i,n) of thechannel characteristic of the even-numbered subcarrier and an estimationvalue Aj(i,n+1) of the channel characteristic of the subcarrier havingthe next number have a high positive correlation.

<Three Transmitting Antennas>

For even-numbered subcarriers of the first received known symbol,L_(M)(n)=L₁(n) holds from equation (3), so Aj(i,n)=hj(i,n) holds fromequation (7). On the other hand, for odd-numbered subcarriers,L_(M)(n)=−L₁(n) holds from equation (3), so Aj(i,n)=−hj(i,n) holds fromequation (7), i.e., the phase of this characteristic is inverted fromthat of the actual channel characteristic. Since an even-numberedsubcarrier and an adjacent subcarrier having the next number aretransmitted from the same antenna, an estimation value Aj(i,n) of thechannel characteristic of the even-numbered subcarrier and an estimationvalue Aj(i,n+1) of the channel characteristic of the subcarrier havingthe next number have a high correlation. However, Aj(i,n+1) correspondsto an odd-numbered subcarrier, and its characteristic has a phaseinverted from that of the actual channel. Therefore, the estimationvalues of these subcarriers have a high negative correlation.

For even-numbered subcarriers of the second received known symbol,L_(M)(n)=L₁(n) holds from equation (3), so Aj(i,n)=hj(i,n) holds fromequation (7). On the other hand, for odd-numbered subcarriers,L_(M)(n)=−L₁(n) holds from equation (3), so Aj(i,n)=−hj(i,n) holds fromequation (7), i.e., the phase of this characteristic is inverted. Sincean even-numbered subcarrier and an adjacent subcarrier having the nextnumber are transmitted from the same antenna, an estimation valueAj(i,n) of the channel characteristic of the even-numbered subcarrierand an estimation value Aj(i,n+1) of the channel characteristic of thesubcarrier having the next number have a high correlation. However,Aj(i,n+1) corresponds to an odd-numbered subcarrier, and itscharacteristic has a phase inverted from that of the actual channel.Therefore, the estimation values of these subcarriers have a highnegative correlation.

In the third received known symbol, L_(M)(n)=L₁(n) holds from equation(3), so Aj(i,n)=hj(i,n) holds from equation (7). Since an even-numberedsubcarrier and an adjacent subcarrier having the next number aretransmitted from the same antenna, an estimation value Aj(i,n) of thechannel characteristic of the even-numbered subcarrier and an estimationvalue Aj(i,n+1) of the channel characteristic of the subcarrier havingthe next number have a high positive correlation.

As is apparent from the above description, when there are M transmittingantennas, the channel characteristic of an even-numbered subcarrier,which is estimated from the Mth received symbol, and the channelcharacteristic of an adjacent subcarrier having the next number have ahigh positive correlation value. Accordingly, it can be estimated atthis point there are M antennas.

The algorithm of estimating the number of transmitting antenna in thereceiver shown in FIG. 5 will be described below with reference to FIG.6. First, “1” is set as an initial value in the counter 37 (step S1),and the waveform in the frequency domain of a known symbol received bythe jth antenna is input to a dividing unit 30-j (steps S2 and S3). Thewaveform of each subcarrier input to the dividing unit 30-j is dividedby the known symbol pattern stored in the ROM 34, and thereby convertedinto a channel characteristic. The channel characteristic is stored in amemory 38-j, and input to a correlator 35-j (step S4).

Then, the correlator 35-j obtains a correlation value of the estimationvalues of the channel characteristics of an even-numbered subcarrier andodd-numbered subcarrier. When the ith known symbol is received, thiscorrelation calculation is defined as follows.(Correlation value)=Aj(i,0)*Aj(i,1)+Aj(i,2)*Aj(i,3)+Aj(i,4)*Aj(i,5)+.Aj(i,N−2)*Aj(i,N−1)where a*b is a calculation of multiplying a by the complex conjugate ofb.

If the correlation value calculated by the correlator 35-j is positive(step S5), the determination unit 36 determines that the currentlyreceived symbol is the last known symbol, and estimates the number oftransmitting antennas on the basis of the number of known symbolpatterns received up to this point and counted by the counter 37 (stepS6). Since the number of known symbols is equal to the number oftransmitting antennas, this counter value is an estimation value of thenumber of transmitting antennas. The MIMO signal processing unit 39reconstructs the data symbols by using the number of transmittingantennas thus estimated.

On the other hand, if in step S5 the correlation value is not positive,the counter 35 is incremented (step S7), and the channel characteristic,which is calculated by the dividing unit 30-j and stored in the memory38-j, and which corresponds to the odd-numbered subcarrier, ismultiplied by −1. The product is stored in the memory 38-j again, andthe next known symbol is received (step S8). The operations in steps S3to S8 are repeated whenever a new known symbol is received.

If a plurality of receiving antennas are used, the following methods arealso possible.

(a) Only when correlation values are positive for all the receivingantennas, it is determined that the end of a known symbol is detected,and the number of transmitting antennas is determined.

(b) Correlation values calculated from all the receiving antennas areadded, and, if the total correlation value is positive, it is determinedthat the end of a known symbol is detected, and the number oftransmitting antennas is determined.

Although in method (a) the conditions are severer, the number oftransmitting antennas can be reliably detected if the conditions aremet.

In the first embodiment as described above, the number of transmittingantennas can be estimated, without any notification of the number oftransmitting antennas from the transmitting side, while channelestimation of each antenna is performed by using a known symbol.

Second Embodiment

A method of transmitting known symbols for channel estimation accordingto the second embodiment will be described below with reference to FIGS.7 to 9. FIGS. 7 to 9 illustrate the structures of radio framescontaining preambles when there are, respectively, one, two, and threetransmitting antennas which simultaneously transmit known symbols.

In the case of FIG. 7 (M=1: one antenna), the combinations of thenumbers of subcarriers, in which information of one known symboltransmitted from antenna 1 exists, and information values carried onthese subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th} subcarriers, information values are{L₁(0), L₁(1), L₁(2), L₁(3), L₁(4), L₁(5), L₁(6), L₁(7), L₁(8), L₁(9),L₁(10), and L₁(11)}

In the case of FIG. 8 (M=2: two antennas), the combinations of thenumbers of subcarriers, in which information of two known symbolstransmitted from antennas 1 and 2 exists, and information values carriedon these subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, 3rd, 4th, and 5th)}subcarriers, information values are {L₁(0), −L₁(1), L₁(2), −L₁(3),L₁(4), and −L₁(5)}

Antenna 1: second known symbol: in {6th, 7th, 8th, 9th, 10th, and 11th}subcarriers, information values are {L₁(6), L₁(7), L₁(8), L₁(9), L₁(10),and L₁(11)}

Antenna 2: first known symbol: in {6th, 7th, 8th, 9th, 10th, and 11th)}subcarriers, information values are {L₁(6), −L₁(7), L₁(8), −L₁(9),L₁(10), and −L₁(11)}

Antenna 2: second known symbol: in {0th, 1st, 2nd, 3rd, 4th, and 5th}subcarriers, information values are {L₁(0), L₁(1), L₁(2), L₁(3), L₁(4),and L₁(5)}

In the case of FIG. 9 (M=three: three antenna), the combinations of thenumbers of subcarriers, in which information of three known symbolstransmitted from antennas 1, 2, and 3 exists, and information valuescarried on these subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, and 3rd)} subcarriers,information values are {L₁(0), −L₁(1), L₁(2), and −L₁(3)}

Antenna 1: second known symbol: in {4th, 5th, 6th, and 7th)}subcarriers, information values are {L₁(4), −L₁(5), L₁(6), and −L₁(7)}

Antenna 1: third known symbol: in {8th, 9th, 10th, and 11th)}subcarriers, information values are {L₁ (8), L₁(9), L₁ (10), and L₁(11)}

Antenna 2: first known symbol: in {4th, 5th, 6th, and 7th)} subcarriers,information values are {L₁(4), −L₁(5), L₁(6), and −L₁(7)}

Antenna 2: second known symbol: in {8th, 9th, 10th, and 11th}subcarriers, information values are {L₁(8), −L₁(9), L₁(10), and −L₁(11)}

Antenna 2: third known symbol: in {0th, 1st, 2nd, and 3rd)} subcarriers,information values are {L₁(0), L₁(1), L₁(2), and L₁(3)}

Antenna 3: first known symbol: in {8th, 9th, 10th, and 11th}subcarriers, information values are {L₁(8), −L₁(9), L₁(10), and −L₁(11)}

Antenna 3: second known symbol: in {0th, 1st, 2nd, and 3rd} subcarriers,information values are {L₁(0), −L₁(1), L₁(2), and −L₁(3)}

Antenna 3: third known symbol: in {4th, 5th, 6th, and 7th)} subcarriers,information values are {L₁(4), L₁(5), L₁(6), and L₁(7)}

As shown in FIGS. 7 to 9, similar to the known symbol of the firstembodiment, the known symbol according to the second embodiment uses twoadjacent subcarriers, and different subcarriers are used for differentantennas. Also, in the known symbol transmitted last, the sameinformation as the known symbol when there is one antenna as shown inFIG. 7 is carried on each subcarrier. In each known symbol transmittedbefore the last known symbol, the same information as the known symbolwhen there is one antennas is carried on even-numbered subcarriers, andphase-inverted information of the known symbol when there is one antennais carried on odd-numbered subcarriers. Therefore, as in the firstembodiment, the number of transmitting antennas can be estimated byusing the receiver shown in FIG. 5.

Third Embodiment

A method of transmitting known symbols for channel estimation accordingto the third embodiment will be described below with reference to FIGS.10 to 12. FIGS. 10 to 12 illustrate the structures of radio framescontaining preambles when there are, respectively, one, two, and threetransmitting antennas which simultaneously transmit known symbols.

In the case of FIG. 10 (M=1: one antenna), the combinations of thenumbers of subcarriers, in which information of one known symboltransmitted from antenna 1 exists, and information values carried onthese subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th}subcarriers, information values are{L₁(0), L₁(1), L₁(2), L₁(3), L₁(4), L₁(5), L₁(6), L₁(7), L₁(8), L₁(9),L₁(10), and L₁(11)}

In the case of FIG. 11 (M=2: two antennas), the combinations of thenumbers of subcarriers, in which information of two known symbolstransmitted from antennas 1 and 2 exists, and information values carriedon these subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th} subcarriers, information values are{L₁(0), −L₁(1), L₁(2), −L₁(3), L₁(4), −L₁(5), L₁(6), −L₁(7), L₁(8),−L₁(9), L₁(10), and −L₁(11)}

Antenna 2: second known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th) subcarriers, information values are{L₁(0), L₁(1), L₁(2), L₁(3), L₁(4), L₁(5), L₁(6), L₁(7), L₁(8), L₁(9),L₁(10), and L₁(11)}

In the case of FIG. 12 (M=3: three antennas), the combinations of thenumbers of subcarriers, in which information of three known symbolstransmitted from antennas 1, 2, and 3 exists, and information valuescarried on these subcarriers are as follows.

Antenna 1: first known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th)} subcarriers, information values are{L₁(0)-L₁(1), L₁(2), −L₁(3), L₁(4), −L₁(5), L₁(6), −L₁(7), L₁(8),−L₁(9), L₁(10), and −L₁(11)}

Antenna 2: second known symbol: in {0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th)} subcarriers, information values are{L₁(0), −L₁(1), L₁(2), −L₁(3), L₁(4), −L₁(5), L₁(6), −L₁(7), L₁(8),−L₁(9), L₁(10), and −L₁(11)}

Antenna 3: third known symbol: in (0th, 1st, 2nd, 3rd, 4th, 5th, 6th,7th, 8th, 9th, 10th, and 11th) subcarriers, information values are{L₁(0), L₁(1), L₁(2), L₁(3), L₁(4), L₁(5), L₁(6), L₁(7), L₁(8), L₁(9),L₁(10), and L₁(11)}

As shown in FIGS. 10 to 12, similar to the known symbols of the firstand second embodiments, the known symbol according to the thirdembodiment uses two adjacent subcarriers, and different subcarriers areused for different antennas. Also, in the known symbol transmitted last,the same information as the known symbol when there is one antenna asshown in FIG. 10 is carried on each subcarrier. In each known symboltransmitted before the last known symbol, the same information as theknown symbol when there is one antenna is carried on even-numberedsubcarriers, and phase-inverted information of the known symbol whenthere is one antenna is carried on odd-numbered subcarriers. Therefore,as in the first and second embodiments, the number of transmittingantennas can be estimated by using the receiver shown in FIG. 5.

Fourth Embodiment

A method of transmitting known symbols for channel estimation accordingto the fourth embodiment will be described below with reference to FIGS.13 to 15. FIGS. 13 to 15 illustrate the structures of radio framescontaining preambles when there are, respectively, one, two, and threetransmitting antennas which simultaneously transmit known symbols.

In the first to third embodiments, channels and the number oftransmitting antennas can be estimated by receiving M known symbols fromM transmitting antennas. In contrast, to raise the accuracy of channelestimation, FIGS. 13 to 15 assume the transmission of 2M known symbols.

In the case of FIG. 13 (M=1:one antenna), the combinations of thenumbers of subcarriers, in which information of two known symbolstransmitted from antenna 1 exists, and information values carried onthese subcarriers are as follows.

Antenna 1: first and second known symbols: in {0th, 1st, 2nd, 3rd, 4th,5th, 6th, 7th, 8th, 9th, 10th, and 11th} subcarriers, information valuesare {L₁(0), L₁(1), L₁(2), L₁(3), L₁(4), L₁(5), L₁(6), L₁(7), L₁(8),L₁(9), L₁(10), and L₁(11)}

In the case of FIG. 14 (M=2: two antennas), the combinations of thenumbers of subcarriers, in which information of four known symbolstransmitted from antennas 1 and 2 exists, and information values carriedon these subcarriers are as follows.

Antenna 1: first and second known symbols: in {0th, 1st, 4th, 5th, 8th,and 9th} subcarriers, information values are {L₁(0), −L₁(1), L₁(4),−L₁(5), L₁(8), and −L₁(9)}

Antenna 1: third and fourth known symbols: in {2nd, 3rd, 6th, 7th, 10th,and 11th)} subcarriers, information values are {L₁(2), L₁(3), L₁(6),L₁(7), L₁(10), and L₁(11)}

Antenna 2: first and second known symbols: in {2nd, 3rd, 6th, 7th, 10th,and 11th)} subcarriers, information values are {L₁(2), −L₁(3), L₁(6),−L₁(7), L₁(10), and −L₁(11)}

Antenna 2: third and fourth known symbols: in {0th, 1st, 4th, 5th, 8th,and 9th} subcarriers, information values are {L₁(0), L₁(1), L₁(4),L₁(5), L₁ (8), and L₁ (9)}

In the case of FIG. 15 (M=3:three antennas), the combinations of thenumbers of subcarriers, in which information of six known symbolstransmitted from antennas 1, 2, and 3 exists, and information valuescarried on these subcarriers are as follows.

Antenna 1: first and second known symbols: in {0th, 1st, 6th, and 7th)}subcarriers, information values are {L₁(0), −L₁(1), L₁(6), and −L₁(7)}

Antenna 1: third and fourth known symbols: in {2nd, 3rd, 8th, and 9th}subcarriers, information values are {L₁(2), −L₁(3), L₁(8), and −L₁(9)}

Antenna 1: fifth and sixth known symbols: in {4th, 5th, 10th, and 11th}subcarriers, information values are {L₁(4), L₁(5), L₁(10), and L₁(11)}

Antenna 2: first and second known symbols: in {2nd, 3rd, 8th, and 9th)}subcarriers, information values are {L₁(2), −L₁(3), L₁(8), and −L₁(9)}

Antenna 2: third and fourth known symbols: in {4th, 5th, 10th, and 11th}subcarriers, information values are {L₁(4), −L₁(5), L₁(10), and −L₁(11)}

Antenna 2: fifth and sixth known symbols: in {0th, 1st, 6th, and 7th)}subcarriers, information values are {L₁(0), L₁(1), L₁(6), and L₁(7)}

Antenna 3: first and second known symbols: in {4th, 5th, 10th, and11th)} subcarriers, information values are {L₁(4), −L₁(5), L₁(10), and−L₁(11)}

Antenna 3: third and fourth known symbols: in {0th, 1st, 6th, and 7th)}subcarriers, information values are {L₁(0), −L₁(1), L₁(6), and −L₁(7)}

Antenna 3: fifth and sixth known symbols: in {2nd, 3rd, 8th, and 9th)}subcarriers, information values are {L₁(2), L₁(3), L₁(8), and L₁(9)}

In the fourth embodiment as described above, identical known symbols arecontinuously transmitted, and this lowers the transmission efficiencybecause the number of known symbols increases. However, the receivingside can reduce the influence of noise by averaging known symbols havingthe same pattern as follows, and thereby can increase the accuracy ofestimation of the number of transmitting antennas and channelestimation.

A receiver according to the fourth embodiment has an arrangement asshown in FIG. 16 in order to perform estimation as described above. InFIG. 16, the same reference numerals as in FIG. 5 denote the same parts.FIG. 16 differs from FIG. 5 in that buffers 41-1 to 41-M and averagingunits 42-1 to 42-M are added to the arrangement of the receiver shown inFIG. 5.

Referring to FIG. 16, OFDM signals of an RF band transmitted from atransmitter are received by a plurality of receiving antennas 30-1 to30-M. The OFDM received signals from the receiving antennas 30-1 to 30-Mare input to a receiving unit 31.

In the receiving unit 31, the input OFDM signals from the receivingantennas 30-1 to 30-M are amplified by a low-noise amplifier (LNA), andconverted (down-converted) into a base band. In addition, thesefrequency-converted signals are converted into digital signals by ananalog-to-digital (A/D) converter, and a guard interval (GI) is removed.

Of these output signals having undergone GI removal, signals having atime section corresponding to odd-numbered received known symbols arestored in the buffers 41-1 to 41-M. The averaging units 42-1 to 42-Mcalculate the average of these known symbols and known symbols receivedin the next time section. The output signals are input to FFT units 32-1to 32-M. The FFT units 32-1 to 32-M transform the waveform signals inthe time domain into waveform signals in the frequency domain, i.e., thewaveforms of individual subcarriers. Of the output signals from the FFTunits 32-1 to 32-M, signals of data symbol sections are input to an MIMOsignal processing unit 39.

On the other hand, of the output signals from the FFT units 32-1 to32-M, signals of preambles, particularly, known symbol sections areinput to dividing units 33-1 to 33-M. The waveforms of the individualsubcarriers input to these dividing units are divided by a known symbolpattern stored in a ROM 34, and thereby converted into estimation valuesof the channel characteristics. These estimation values are stored inmemories 38-1 to 38-M and input to correlators 35-1 to 35-M. The knownsymbol pattern stored in the ROM 34 is the same as the first knownsymbol pattern in the case of FIG. 13 (one antenna).

The correlators 35-1 to 35-M calculate a correlation value by using thechannel characteristic estimation values, and input the correlationvalue to a determination unit 36. If the input correlation value isnegative, the determination unit 36 increments a counter 37, andreceives the next known symbol. If the input correlation value ispositive, the determination unit 36 determines that the currentlyreceived symbol is the last known symbol, and outputs the currentcounter value of the counter 37 as an estimation value of the number oftransmitting antennas to an MIMO signal processing unit 39.

The MIMO signal processing unit 39 performs an MIMO signal receivingprocess, e.g., maximum likelihood estimation, on the data symbol sectionsignals from the FFT units 32-1 to 32-M in accordance with the channelestimation values from the memories 38-1 to 38-M, and the estimationvalue of the number of transmitting antennas from the counter 37.Channel decoding is performed on the signals having undergone this MIMOsignal receiving process, thereby reconstructing transmitted data 40.

Fifth Embodiment

In the first to fourth embodiments described above, for each knownsymbol except for a known symbol to be transmitted last of a knownsymbol sequence transmitted from each antenna, the phase of knowninformation to be transmitted on one of two adjacent subcarriers isinverted. The phase of known information of the known symbol to betransmitted last is not inverted, indicating that this known symbol isthe end of the known symbol sequence.

That is, the correlation value of the known symbol at the end of theknown symbol sequence is a positive value. The number of transmittingantennas is estimated on the basis of the number of known symbols havingnegative correlation values and received before the known symbol havinga positive correlation value is received.

The following method, however, is also possible. That is, for a knownsymbol to be transmitted last of a known symbol sequence transmittedfrom each antenna, the phase of known information to be transmitted onone of two adjacent subcarriers is inverted. The phase of knowninformation of each known symbol except for the known symbol to betransmitted last is not inverted. In this method, the correlation valueof the known symbol at the end of the known symbol sequence is anegative value. The number of transmitting antennas is estimated on thebasis of the number of known symbols having positive correlation valuesand received before the known symbol having a negative correlation valueis received.

For example, in FIG. 3, information values {L₁(0), L₁(1), L₁(4), L₁(5),L₁(8), and L₁(9)} are allocated to the (0th, 1st, 4th, 5th, 8th, and9th} subcarriers of the first known symbol of antenna 1, informationvalues {L₁(2), −L₁(3), L₁(6), −L₁(7), L₁(10), and −L₁(11)} are allocatedto the {2nd, 3rd, 6th, 7th, 10th, and 11th} subcarriers of the secondknown symbol of antenna 1. Also, information values {L₁(2), L₁(3),L₁(6), L₁(7), L₁(10), and L₁(11)} are allocated to the {2nd, 3rd, 6th,7th, 10th, and 11th)} subcarriers of the first known symbol of antenna2, and information values {L₁(0), −L₁(1), L₁(4), −L₁(5), L₁(8), and−L₁(9)} are allocated to the {0th, 1st, 4th, 5th, 8th, and 9th}subcarriers of the second known symbol of antenna 2.

This similarly applies to FIG. 4. That is, the phases of knowninformation to be carried on odd-numbered subcarriers of the third knownsymbols of antennas 1, 2, and 3 are inverted, and the phases of knowninformation to be carried on subcarriers of the first and second knownsymbols are not inverted.

The same applies to FIGS. 7 to 9, 10 to 12, and 13 to 15.

In this case, the arrangement of the transmitter is the same as FIG. 1,and the arrangement of the receiver is the same as FIG. 5.

Only the difference from the first to fourth embodiments will beexplained below.

In the algorithm of the transmitting antenna count estimating sequenceshown in FIG. 6 performed by the receiver shown in FIG. 5, theprocessing from step S5 is different. That is, as shown in FIG. 18, ifthe correlation value calculated by a correlator 35-j is negative (stepS11), the determination unit 36 determines that the currently receivedsymbol is the last known symbol, and estimates the number oftransmitting antennas on the basis of the number of known symbolpatterns received so far and counted by the counter 37 (step S12). Sincethe number of known symbols of a known symbol sequence transmitted fromeach antenna is equal to the number of transmitting antennas, thiscounter value is an estimation value of the number of transmittingantennas. The MIMO signal processing unit 39 reproduces the data symbolsby using the estimated number of transmitting antennas.

On the other hand, if the correlation value is positive in step S1, thecounter 35 is incremented (step S13), and the channel characteristiccorresponding to odd-numbered subcarriers, which is calculated by adividing unit 30-j and stored in a memory 38-j, is multiplied by −1. Thevalue is stored in the memory 38-j again, and the next known symbol isreceived (step S14). After that, the operations in steps S3, S4, and S11to S14 are repeated whenever a new known symbol is received.

As described above, the transmitter according to the first to fifthembodiments transmits a plurality of known symbol sequences eachincluding a plurality of known symbols by using a plurality of antennas.Each known symbol is carried on a plurality of subcarriers.Inversion/non-inversion of the phase of known information of each knownsymbol carried on one of two adjacent subcarriers of the subcarriers iscontrolled in accordance with the number of the known symbols and timeposition of the each of the known symbols;

In the known symbol sequence according to the first to thirdembodiments, subcarrier arrangements (allocations) of known symbols,i.e., the arrangements of a plurality of subcarriers on which knowninformation of each of the known symbols carried, are different.

Also, the known symbol sequence according to the fourth embodimentincludes identical known symbols whose known information and subcarrierarrangements on which the known information of each of the identicalknown symbols carried are the same.

In the transmitter according to the first to fourth embodiments, ofknown symbols in a known symbol sequence transmitted from each antenna,the phase of known information of each of the known symbols carried onone of two adjacent subcarriers of each known symbol except for the lastknown symbol is inverted, and the phase of known information of thislast known symbol carried on each subcarrier is not inverted. Therefore,the receiver estimates the channel characteristic of a known symbolsequence received by each antenna. If the correlation between thechannel estimation values of adjacent subcarriers has a positive value,the receiver determines that the end of the known symbol sequence isdetected. The receiver can easily estimate the number of transmittingantennas from the number of known symbols received so far in each ofwhich the correlation between the channel estimation values of adjacentsubcarriers has a negative value.

In the transmitter according to the fifth embodiment, of known symbolsin a known symbol sequence transmitted from each antenna, the phase ofknown information of each known symbol carried on one of two adjacentsubcarriers of the last known symbol is inverted, and the phase of knowninformation of each known symbol except for this last known symbol isnot inverted. Therefore, the receiver estimates the channelcharacteristic of a known symbol sequence received by each antenna. Ifthe correlation between the channel estimation values of adjacentsubcarriers has a negative value, the receiver determines that the endof the known symbol sequence is detected. The receiver can easilyestimate the number of transmitting antennas from the number of knownsymbols received so far in each of which the correlation between thechannel estimation values of adjacent subcarriers has a positive value.

In each of the above embodiments as explained above, the receiving sidecan easily estimate the number of antennas used in transmission.

1. A wireless communication apparatus comprising: a plurality ofantennas; a first transmitting unit configured to transmit a pluralityof known symbol sequences each including a plurality of known symbols byusing the antennas, each of the known symbols having subcarrierarrangement on which plural known information of the each of the knownsymbols are carried on, and inversion/non-inversion of a phase of knowninformation on one of two adjacent subcarriers of the subcarrierarrangement being controlled in accordance with the number of the knownsymbols and time position of the each of the known symbols; and a secondtransmitting unit configured to transmit data symbols by using theantennas after the known symbol sequences are transmitted.
 2. Anapparatus according to claim 1, wherein known symbols simultaneouslytransmitted from different antennas have different subcarrierarrangements.
 3. An apparatus according to claim 1, wherein the phase ofthe known information on one of two adjacent subcarriers of thesubcarrier arrangement of the last known symbol of the known symbols isnot inverted, and the phase of the known information of on one of twoadjacent subcarriers of the subcarrier arrangement of each known symbolof the known symbols except for the last known symbol is inverted.
 4. Anapparatus according to claim 1, wherein the phase of the knowninformation on one of two adjacent subcarriers of the subcarrierarrangement of the last known symbol of the known symbols is inverted,and the phase of the known information on one of two adjacentsubcarriers of the subcarrier arrangement of each known symbol of theknown symbols except for the last known symbol is not inverted.
 5. Anapparatus according to claim 1, wherein the known symbols included ineach of the known symbol sequences have different subcarrierarrangements.
 6. An apparatus according to claim 1, wherein each of theknown symbol sequences includes identical known symbols whose subcarrierarrangements are the same.
 7. An apparatus according to claim 1, whereinthe first transmitting unit includes: memory to store a plurality ofknown symbol patterns having different subcarrier arrangements; timinggenerating unit configured to generate a timing signal indicating atiming at which the known symbol is to be transmitted; and selector toselect a known symbol pattern to be used in the known symbol from theknown symbol patterns in accordance with the timing signal.
 8. Awireless communication apparatus comprising: receiving unit configuredto receive a plurality of known symbol sequences and subsequent datasymbols transmitted by a plurality of antennas, each of the known symbolsequence including a plurality of known symbols, each of the knownsymbols having subcarrier arrangement on which plural known informationof the each of the known symbols are carried on, andinversion/non-inversion of a phase of known information on one of twoadjacent subcarriers of the subcarrier arrangement being controlled inaccordance with the number of the known symbols and time position of theeach of the known symbols; a first calculating unit configured tocalculate each channel estimation value corresponding to each of thesubcarriers, from each known symbol received; a second calculating unitconfigured to calculate a correlation value between two channelestimation values corresponding to the two adjacent subcarriers, toobtain each correlation value corresponding to the each known symbolreceived; an estimating unit configured to estimate the number ofantennas based on the each correlation value and the number of knownsymbols received; and a processing unit configured to process the datasymbols received by using the each channel estimation value and thenumber of antennas estimated.
 9. An apparatus according to claim 8,wherein the estimating unit estimates the number of antennas based onthe number of known symbols whose correlation values are negative, andeach of which is received before a known symbol whose correlation valueis positive is received.
 10. An apparatus according to claim 8, whereinthe estimating unit estimates the number of antennas based on the numberof known symbols whose correlation values are positive, and each ofwhich is received before a known symbol whose correlation value isnegative is received.
 11. An apparatus according to claim 8, whereineach of the known symbol sequences includes identical known symbolswhose subcarrier arrangements are the same; and the first calculatingunit includes a averaging unit configured to average waveforms in thefrequency domain corresponding to the identical known symbols, to obtaina waveform averaged; and calculates the each channel estimation valuefrom the waveform averaged.